For cake: Preheat oven to 350°F. Butter and flour two 9-inch-diameter cake pans with 2-inch-high sides; line bottoms with rounds of parchment paper.

Sift first 4 ingredients into medium bowl. Using electric mixer, beat butter in large bowl until fluffy. Gradually add sugar, beating until blended, scraping down sides of bowl occasionally. Beat in lemon juice and peel, then eggs 1 at a time. Continue to beat until well blended. Beat in dry ingredients in 4 additions alternately with buttermilk in 3 additions. Fold in berries. Transfer batter to pans. Bake cakes until tester inserted into center comes out clean, about 40 minutes. Cool cakes in pans on racks.

For frosting: Stir white chocolate in top of double boiler set over barely simmering water until almost melted. Remove from over water and stir until smooth. Cool to lukewarm. Beat cream cheese and butter in large bowl until blended. Beat in lemon juice, then cooled white chocolate. Add powdered sugar until frosting is smooth and moderately stiff.

Turn cakes out onto work surface. Peel off parchment. Place 1 cake layer, flat side up, on platter. Spread with 1 cup frosting. Top with second cake layer, flat side down. Spread remaining frosting over top and sides of cake. Garnish with additional blueberries and lemon slices, if desired. (Can be made 1 day ahead. Cover with cake dome; refrigerate. Let stand at room temperature 1 hour before serving.)